1. Field of the Invention
This invention generally relates to methods and systems for variable polarization wafer inspection. Certain embodiments relate to optimizing one or more variable polarizing components for nuisance suppression.
2. Description of the Related Art
The following description and examples are not admitted to be prior an by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
Currently, in dark field wafer inspection tools, polarizing components are implemented in discrete configurations in the detection channels of the tools. Two polarization directions are available: S and P. On side channels, P is defined as the polarization direction perpendicular to the scattering plane (defined by the normal line of the scattering surface and the optical axis of the collection optics), S is defined as the polarization parallel to the scattering plane and perpendicular to the optical axis of the collection optics. On the center channel, there is no well-defined scattering plane, S is defined to be parallel with the S polarization of the illumination, and P is defined to perpendicular to S and the optical axis of center channel collection optics. In this definition, P of all three channels are parallel to one another. It is noted that these definitions are different from the academic definitions of S and P polarization of scattered light and will be used throughout this document. There is also a “none analyzer” that is simply a piece of non-polarizing silica glass.
There are also three discrete polarizations that could be selected for illumination: P, S, and C (circular). So, in total, 9 polarization combinations could be selected for any given combination of an illumination channel and a detection channel on current systems. During inspection recipe setup, polarization combinations are evaluated by comparing the signal-to-noise ratio (SNR) of known defects. The polarization combination that results in the best signal to noise ratio (SNR) is chosen for the inspection recipe.
In practice, it has been shown that polarization plays a critical role in wafer inspection because of the highly polarized nature of light scattered from defects and wafer patterns. On the most difficult wafers, cross polarizations, P-S and S-P, are frequently used in illumination and detection to suppress wafer noise and enhance the SNR of defects.
The discrete polarization configurations used on current inspection systems have two limitations in defect detection. First, when the illumination is set to P polarization, S polarization for detection, in general, will not result in minimum noise from the wafer surface or pattern scattering as shown in experimental data and modeling. This limitation is an inherent result of spatial symmetry in the optical layout of illumination and collection optics, not a result of alignment or component imperfection. Hence, the SNR of defects is not truly optimized as intended.
Second, on some wafers, there are enormous amounts of nuisance defects (also referred to herein as nuisances) coexisting with the defects of interest (DOIs). On those wafers, the nuisance defects are real scattering sources but of no interest from the point of view of semiconductor process control. One example of this kind of nuisance defects is previous layer defects. Multiple types of wafers have shown this kind of issue. In general, different types of defects tend to exhibit different polarization dependencies. One potential solution to distinguish the DOIs from nuisance defects is to use polarization. But, the discrete polarization configurations on current systems significantly limit the capability of differentiating defects.
Accordingly, it would be advantageous to develop methods and systems for suppressing nuisance defects in dark field wafer inspection.